from philadelphia. It has happened again. An apparently random, unprovoked subway attack, ending with a woman thrown to the tracks. You see it right there. Thankfully, she survived. And abc's john muller has the details. Reporter: This surveillance video taken inside philadelphia's subway system is stunning and hard to watch. A brutal attack on an unsuspecting victim, thrown screaming on to the tracks of a train. It begins innocently enough. A man and woman wait for the subway after 3:00 in the afternoon. Then, the man approaches and asks the woman to borrow a lighter. Returning it, he lunges, choking her and attacking her with a flurry of punches. Then, the unthinkable. The suspect grabs her ankles, dragging her across the platform, and tossing her on to the tracks, before casually reaching into her purse. Picked up her cell phone and strolled out of the station. Reporter: Overnight, after a citywide manhunt, police have arrested william clark and plan to charge him with aggravated assault and robbery. He was ablended, still in possession of the victim's phone. Reporter: Miraculously, police say the young woman was able to climb to safety, before a train reached her, suffering only minor injuries. What police don't know is why an innocent moment waiting for a train became such a nightmare. For "good morning america," john muller, abc news, new york.

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